Cracking! 19 April 2023Learning objective: To know what cracking is and why it’s so useful!

Starter Name fractions from the fractional distillation column which are MOST useful? Explain

Fractions

CrackingCrackingAll (Grade C) Most (Grade B) Some ( Grade A/A*)

State the reasons for cracking and how it is carried out

Describe the conditions for cracking and how to test for alkenes

Analyse word equations to write a balanced symbol equation for cracking

Draw the molecular structure and write the chemical formula for:-

A ) Decane. Now, split it up into Octane and ethene. What is this process called?

KEYWORD: Cracking

Learning objective: To know what cracking is and why it’s so useful!

Learning objective: To know what cracking is and why it’s so useful!

Cracking HydrocarbonsCracking HydrocarbonsLarge hydrocarbons are broken into smaller

molecules using heat and a catalyst.This process is known as catalytic cracking.The small molecules produced are then separated

by distillation.

CatalyticcrackerHeat to

vaporise

Distillationtower

pressure

Big Molecules

Sm

alle

r m

ole

cule

s

Molecules break up

Catalytic CrackingCatalytic CrackingIn the catalytic cracker long chain

molecules are split apart or ‘cracked’. An example of such a reaction is:

C8H18 C6H14 + C2H4

C CH

H

H

H+

ethene

C CH

H

H

H H

C C C C C C

H

H

H

H

H

H

H

H

H

H

H

H

H

Octane

C CH

H

H

H H

C C C C

H

H

H

H

H

H

H

H

H

hexane

Etheneis usedto makeplastics

Heat pressure

catalyst

Used as a fuel

Fuels made from oil mixtures containing large hydrocarbon molecules are not efficient. They do not flow easily and are difficult to ignite. Crude oil often contains too many large hydrocarbon molecules and not enough small hydrocarbon molecules to meet demand - this is where cracking comes in.

Cracking is a useful process that converts useless heavy fractions into smaller useful small ones

Cracking allows large hydrocarbon molecules to be broken down into smaller, more useful hydrocarbon molecules. Fractions containing large hydrocarbon molecules are vaporised and passed over a hot catalyst. This breaks chemical bonds in the molecules, and forms smaller hydrocarbon molecules. Cracking is an example of a thermal decomposition reaction.

Some of the smaller molecules formed by cracking are used as fuels, and some of them are used to make polymers in plastics manufacture.

Conditions for cracking:

1.High temperature (300oC)2.Use a catalyst (aluminum oxide)

Demo of CrackingDemo of Cracking

Watch the demo that your teacher will carry out. While the practical is being carried out, make careful observations to be able to answer these questions. Label the apparatus1 ) What are the conditions needed for cracking to take place?2) What type of reaction is taking place? 3) Describe the test for alkenes. State the chemical used and the result you should expect.

Learning objective: To know what cracking is and why it’s so useful!

Catalysts are special

substances that

make reactions go

faster

But they are still present at the end of the reaction !

Catalyst allow reactions to

happen at lower temperatures

and pressures that otherwise

would be needed

Processes are therefore cheaper to carry out

Some catalysts are expensive metals like Platinum

It doesn’t matter though, as catalysts are never used up!

Catalysts in the body and in biological

situations are called enzymes

Alkanes : Supply and DemandAlkanes : Supply and Demand

0

5

10

15

20

25

30

35

40

1 4 8 12 16 20 >24

Length of carbon chain

% in crude oil

Demand

Methane gas Octane – found in the petrol fraction

Long chains – found in the residue fraction

• What is the relationship between the amounts of the alkanesin crude oil and their demand?• Can you think of a way of sorting out this problem?

Q

Cracking...what’s it all about?Cracking...what’s it all about?

Using the molymods, try to model the process of cracking

Try to answer the questions below:

Q1 – What conditions are needed for this process

Q2 – Why are the shorter chains more useful?

Q3 – What can be done with “leftovers”?

EXT: What process is

cracking the opposite of?

PolymersAlkenes can be used to make polymers. Polymers are very large molecules made when many smaller molecules join together, end-to-end. The smaller molecules are called monomers. Lots of monomer molecules → a polymer molecule.

polymer usepolyethene plastic bags and bottles

polypropene crates and ropes

polychloroethenewater pipes and insulation on electricity cables

Examples of polymers and their uses

chemical structure

ball-and-stick model

Structural formula

etheneC2H4

PropeneC3H6

What is the formula for Alkenes?

Learning objective: To know what cracking is and why it’s so useful!

What products could be made from cracking C8H18?

Learning objective: To know what cracking is and why it’s so useful!

long-chain alkane short-chain alkane + alkene300°C

aluminium oxide

Ethene

C

H

H

C

H

H

Ethene is the first member of the alkene series

Ethene contains a carbon to carbon double bond

Ethene contains four carbon to hydrogen single bonds

C

H

H

C

H

H

or

Addition reaction – ethene and bromine

C

H

H

C

H

H

Bromine molecules can react with unsaturated hydrocarbons with carbon to carbon double bonds as follows:

Br Br

The bromine atoms from the bromine molecule have added on to the carbon atoms by breaking the carbon to carbon double bond

Bond breaking

Bond formingBond breaking

H

H

H

H

Two new carbon to bromine single bonds formed

Orange-brown coloured bromine solution changes to a colourless solution in this reaction

1,2-dibromoethane is the new product

A catalyst is a substance that is ____________in a reaction, but makes the reaction go __________.

• Catalysts are often expensive metals, e.g. gold or platinum. Why do you think the long-term costs are low?

Catalysts

1. What is cracked?2. What are the products formed during

cracking?3. Name the 2 conditions needed for cracking.4. Name the catalyst involved in cracking5. What is a catalyst?6. Give an example of a catalyst.7. Draw the structural formula when propane

is cracked in methane and …………

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Learning objective: To know what cracking is and why it’s so useful!